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  • Proceedings of IOE Graduate Conference, 2017 Volume: 5 ISSN: 2350-8914 (Online), 2350-8906 (Print)

    Design, Fabrication and Testing of Waste Plastic Pyrolysis Plant

    Ajay Jayswal a, Arbind Kumar Sah b, Prabin Pradhananga c, Rohit Sah d, Hari Bahadur Darlami e

    a, b, c, d, e Department of Mechanical Engineering, Pulchowk Campus, Institute of Engineering, Tribhuvan University, Nepal Corresponding Email: a ajayjayswal604@gmail.com, b arvindsah100@gmail.com, c 2prabinp2@gmail.com, d sahrohit153@gmail.com, e haridarlami@gmail.com

    Abstract In recent decades, there has been a dramatic increment in plastic consumption. Used plastic is one of the major wastes in many countries including Nepal. A lot of money is spent in land filling to process plastic wastes which can pose a threat to environment in long run. The incineration of plastic wastes leads to severe air pollution. Plastic pyrolysis process is a widely used technique to handle plastic wastes in many foreign countries. It is a new technology in Nepalese context. It involves melting plastic wastes, vaporizing them, condensing the vapor and distilling to obtain fuel. In the pyrolysis reactor, plastics are heated, vaporized and the vapor thus produced is passed to shell and tube condenser for condensation. The liquid thus obtained is called pyrolysis oil and char remains in pyrolysis reactor as residue. The yields depend on various factors like plastic type used, cracking temperature of plastic, rate of heating, operation pressure of reactor, type of reactor, residence time, use of catalyst, etc. The plant was designed and modeled in 3D CAD software, Solidworks. Batch reactor was employed to pyrolyze Low Density Polyethylene at reactor base temperature of about 600°C and the vapour produced was directed to horizontal, counter-flow shell and tube condenser. From 10 kg of plastics, the plant yielded 6.63 liters of pyrolysis oil and 2.236 kg of char, on average, in the cost of 3.169 kg of LPG gas.

    Keywords Waste Plastic – Plastic Pyrolysis – Pyrolysis Oil – Shell and Tube Condenser

    1. Introduction

    Plastics are one of the most commonly used materials in our daily life. They contribute to make our life convenient. They are widely used in packaging and manufacture of products including electronic, automotive, etc [1]. Plastics have light weight and can be simply formed. They can be reused and help to conserve natural resources [2]. In fact, plastics have been used to replace metals and wood. Resultantly, plastic consumption has skyrocketed.

    Plastic was invented in 1862 by Alexander Parkes [3]. They are formed by polymerization and have high molecular mass. Other substances may be present in plastics besides polymers to minimize costs and to enhance performance [4]. Desired shape can be given to these polymers by molding or by extrusion [5].

    Plastic pyrolysis involves heating and degradation of plastic polymers at temperatures between 350°C and 900°C in an oxygen deficient environment [2]. This

    results in the formation of carbonized solid residue called char, condensible hydrocarbon oil and non-condensible gas with high calorific value [2]. Scheirs et al. [5] stated that gases formed during the pyrolysis of organic material include carbon monoxide, carbon dioxide, water, hydrogen, methane, ethane, ethene, propane, propene, butane, butene, etc. The temperature and rate of heating can be controlled to produce desired solid, liquid and gas products because they have significant influence in pyrolysis process [5]. Yin et al. [6] have considered pyrolysis of waste plastic as one of the outstanding methods of energy regeneration. This is because waste plastic is an important resource of chemicals, gas and liquid fuels [6].

    Mainly there are two types of plastics: thermoplastics and thermosetting polymers. If enough heat is supplied, thermoplastics can be softened and melted repeatedly. On cooling, they are hardened, so that they can be used to form new plastics products. Examples include

    Pages: 275 – 282

  • Design, Fabrication and Testing of Waste Plastic Pyrolysis Plant

    polyethylene, polystyrene, etc [4]. They are recyclable. Thermosetting plastics can be melted and shaped only once. It is not good to repeatedly heat treat such plastics; therefore they remain in solid state after they have been solidified [4]. Examples: epoxy resin, phenol formaldehyde, etc [7]. Society of Plastics Industry (SPI) has divided plastics into the following groups on the basis of application and chemical structure.

    • PET (Polyethylene Terephthalate) • HDPE (High Density Polyethylene) • PVC (Polyvinyl Chloride) • LDPE (Low Density Polyethylene) • PP (Polypropylene) • PS (Polystyrene) • Other

    2. Design Procedure

    2.1 Design of Pyrolysis Reactor

    Determination of the amount of raw materials : The experiment initially aimed towards performing pyrolysis at large scale. For that, the volume occupied by definite mass of plastics at normal market condition was predicted. The volume appeared to be large. Hence, it was chosen to preheat the plastics at low temperature (below melting point) in reactor so that more plastics could be accommodated in reactor. Finally, the volume of reactor was predicted to process 10 kg of plastics.

    Determination of vessel geometry : The pyrolysis reactor was designed to have the vertical cylindrical cross section, called as shell. Heads, which are the end caps of the reactor, were chosen to be kept flat to lessen cost of fabrication, to ensure easy maintenance and to allow for greater rate of heat transfer. The reactor was designed to operate at normal atmospheric pressure. Specific spots were located to attach pressure gauge, to connect piping for guiding vapour to condenser and for the removal of char.

    Analysis of heat required and selection of heating system : The total amount of heat energy required to heat, melt, boil, pyrolyze and vaporize 10 kg LDPE was calculated. The heat transfer rate required and the LPG fuel feed rate required were calculated. LPG stove was selected based on its calorific value and stove efficiency

    to meet our requirements. Water boiling test was done to determine stove efficiency. The plastic vapour production rate was thus calculated and was used later for the design of condenser.

    2.2 Design of Condenser

    Shell and tube condensers are employed for condensation of process vapours and are mostly used in the chemical process industries [8]. Shell and tube condenser with very slight inclination to horizontal was designed to be used in our experiment in order to facilitate proper support and for the liquid oil to flow naturally under the action of gravity for easy collection in the flask. The detailed design procedure for condenser has been given in table 4 in appendix.

    All the 3D modeling and design of pyrolysis reactor, condenser and overall plant were done in Solidworks 2016.

    3. Fabrication and assembly

    3.1 Fabrication of Pyrolysis Reactor

    Figure 1: Pyrolysis Reactor

    Scrap materials from the project done by Acharya et al. [9] were extensively used for fabrication of reactor and condenser. Locally available mild steel of thickness 3 mm was used for the fabrication of heads, shells and conical cap. Fabrication processes of cutting and welding were used [9]. Various specifications of

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    pyrolysis reactor are listed in table 1.

    Table 1: Specifications of Pyrolysis Reactor [9]

    Specifications Value Outer Diameter 500 mm Inner Diameter 494 mm Height of reactor 476 mm Capacity/Volume 91 liters (approx.)

    Two locally available cast iron water pipes of diameters 1.5′′ and 2.5′′ were welded to the reactor, the former at height of 396 mm and the latter, just above the base. Ball valves were connected to each pipe [9]. The pressure gauge was installed in the former and the latter was used as a pathway for char removal. A water pipe of 2.5′′

    was also connected at the side of conical cap to allow plastic vapor to enter the condenser shell. Asbestos gasket and high temperature silicone were used to seal reactor and conical cap. Finally, nuts and bolts were used to connect them. Sensor of electronic temperature measuring instrument was inserted inside the reactor via feed hole.

    3.2 Fabrication of Condenser

    Figure 2: Condenser

    Condenser shell, baffles and cover plate were made from mild steel [9]. The copper tubes were arranged in the tube sheet and then brazed to form tube bundle. Gaskets and high temperature silicone were used to seal cover plate and shell of the condenser. Various specifications of the condenser are enlisted in table 2.

    Table 2: Specifications of Condenser [9]

    Specifications Value Condenser type Shell and Tube Flow type Counter flow Shell side Plastic Vapor Copper tube side Water Copper tube size Outer diameter = 12.7 mm

    Inner diameter = 10.92 mm Tube length per pass 1.5 m No. of passes 1 No. of copper tubes 12 No. of baffles 28 Baffle Spacing 52 mm

    3.3 Assembly

    The reactor and the condenser were connected via cast iron water tube of 2.5′′ diameter. A locally available LPG stove was used as the heating source. Reactor was placed on the stove and a simple furnace made of bricks and mud was constructed on it. The final assembly of the plant is shown in figure 3.

    Figure 3: Waste Plastic Pyrolysis Plant

    4. Testing and Analysis

    Two water boiling tests were conducted to calculate the efficiency of LPG stove. A known mass of water was boiled in pyrolysis reactor by supplying LPG fuel of known heating value. The ratio of theoretical heat absorbed by water to boil plus py